The present invention relates to a light source device which can be used with a liquid crystal display device which requires a flat or planar illuminator.
Some liquid crystal display devices of the time-multiplexed drive type or active matrix type (for example, thin film transistor type) are of the back light type that has a light source disposed on the backside of a liquid crystal display panel to display a clear and bright image thereon.
The illuminator for the liquid crystal display device of the type referred to above may be formed by the light source device disclosed in U.S. Pat. No. 4,059,916. The light source device includes a transparent plate corresponding in size to an associated liquid crystal display panel and a light source disposed adjacent to an end face of the transparent plate. The light source device is characterised in that it utilizes the light source disposed adjacent to the one end face of the transparent plate to provide a uniform surface illumination over a wide area. The illuminator is also characterised in that, because the light source is disposed adjacent to the one end face of the transparent plate, the thickness of the transparent plate can be reduced to insure reduction in the thickness of the illuminator. This is especially advantageous in the case where the illuminator is used with the liquid crystal display device which is inherently required to be of a reduced thickness. The transparent plate is made of a material having a good light transmission characteristic and has a smooth front surface adjacent to the liquid crystal display panel and a rough rear surface which is designed not only to diffuse the light from a light source, but also to reflect the light from the light source to the smooth front surface. The roughened rear surface of the transparent plate has a portion which is beyond a predetermined distance from the end of the transparent plate adjacent to the light source and which is inclined relative to the smooth front surface so that the thickness of the transparent plate is gradually reduced toward the other end of the transparent plate. The inclined surface portion of the roughened rear surface is intended to positively reflect the light from the light source to the smooth front surface so as to compensate for the attenuation of the light which passes through the transparent plate. In other words, the inclined surface portion is designed to prevent reduction in the luminance of the light reflected to the portion of the front surface remote from the light source to insure a uniform luminance of the light over the entire surface area of the front surface.
From the basic research of light source devices in the course of the development of liquid crystal display devices, the inventors have learned that the provision of the inclined surface portion in the roughened rear surface of the transparent plate is insufficient to prevent the reduction in the luminance of the light reflected to the portion of the front surface remote from the light source. From the study by the inventors, it has been found that the inclined surface portion of the roughened rear surface directly reflects the weak light which has been attenuated towards the end of the transparent plate remote from the light source, so that the uniformity of the luminance of the light on the smoothened front surface of the transparent plate is lowered with a resultant disadvantageous reduction in the uniformity of the luminance of the light on the liquid crystal display panel of the liquid crystal display device. This disadvantage is particularly true with the case of large-size liquid crystal display devices.